Conventionally, a technique for visualizing the interior of an object by irradiating an object (article) with radiation represented by X-rays and imaging the spatial distribution (radiation distribution) of radiation components which have been transmitted through the object is used. However, since the radiation generates scattered radiation (scattered rays) inside the object, the scattered radiation (scattered rays) is imaged together with directly transmitted radiation (direct rays) that has been transmitted through the object.
The generation process of scattered rays depends on the type of radiation, and the physical properties, structure, or the like of the object, and scattered rays are normally unpredictable. Hence, in order to obtain a radiation image free from any scattered rays, various measures must be taken.
In a typical method of easily obtaining a radiation image free from any scattered rays, a wall made of a radiation-shielding material such as lead or the like is provided to a radiation image-receiving surface to restrict the angles of radiation components that can reach the image receiving surface, thereby intercepting scattered ray components.
More specifically, in radiography in the medical field, a device called “grid” is inserted between an object such as a human body or the like and the X-ray image-receiving surface to remove X-rays scattered from the object.
The grid is a device formed by alternately arranging, in a predetermined width, an X-ray shielding material such as lead or the like, and an X-ray transparent material such as wood, paper, aluminum, carbon, or the like to have angles in a direction to converge to an X-ray generation source (focal point).
Since the aforementioned grid removes some direct rays, the shade of the grid (the shadow of the grid, to be also referred to as “grid stripes” hereinafter) is observed on the X-ray image receiving surface. However, by adopting an arrangement which satisfies the requirements that “the X-ray shielding and transparent materials are alternately arranged at accurate spatial periods”, “the period is set at a relatively high spatial frequency”, and the like, a disrupted feeling experienced by the X-ray image observer due to the presence of grid stripes on the X-ray image is minimized.
FIG. 39 illustrates the arrangement of a grid 940.
Referring to FIG. 39, reference numeral “910” denotes an X-ray generation source; and “920”, the radiation directions of X-rays. Reference numeral “930” denotes an object such as a human body or the like; and “950”, an X-ray image-receiving surface.
As shown in FIG. 39, the grid 940 normally has a stripe structure in one direction (vertical direction indicated by the arrow in FIG. 39) on a two-dimensional plane due to easy manufacture, and the like.
As a method of reducing (removing) any contrast of the grid stripes, a method of moving only the grid in a direction perpendicular to stripes during X-ray exposure to exploit the integration effect in X-ray exposure on the X-ray image-receiving surface is available.
Note that the radiation image-receiving surface (image-receiving portion) mainly means a radiation film that directly records a radiation distribution on a photosensitive material.
In recent years, as for radiation images for medical use or the like, a method of processing radiation images as digital data is becoming more prevalent than a method of directly forming an image using a radiation film.
For example, a radiation distribution is temporarily converted into an electrical signal (analog signal), which is A/D-converted into numerical value data (digital data). In this manner, the digital radiation image can undergo processes such as filing, an image process, monitor display, and the like with low cost.
However, in case of a digital radiation image, since an image signal must be sampled in a two-dimensional space, a problem of aliasing based on the sampling theorem is notable.
More specifically, in case of a normal image, by setting an appropriate sampling period (shorter period) in a space, aliasing is negligible upon observing an image.
By contrast, in case of a digital radiation image obtained using a grid, a periodic stripe pattern formed by the grid has a very low frequency due to aliasing, or no aliasing is generated due to the sampling period but a low-frequency amplitude variation occurs, thus posing a problem with which the image observer must be concerned.
As a method of removing inappropriate grid stripe patterns due to aliasing or the like, the following methods have been proposed.
A method of converting a radiation image into digital data is roughly classified into two methods, and (method 1) and (method 2) for removing a grid stripe pattern in correspondence with these two methods will be described below.
(Method 1)
In this digital conversion method, a radiation distribution (radiation intensity distribution) is temporarily converted into another energy distribution (that of, e.g., fluorescence), which is scanned to generate an analog video signal (time signal) that spatially samples an image in only one direction. The time signal is A/D-converted based on a separately prepared temporal period.
As an example of this digital conversion method, a method of sequentially converting energy stored in a photostimulable phosphor into light by laser scan, focusing the light, capturing the light as a video signal, and then A/D-converting the video signal is known.
Upon capturing a digital radiation image by the aforementioned digital conversion method, as a method of removing an inappropriate grid stripe pattern due to aliasing or the like, methods described in, e.g., U.S. Pat. Nos. 2,507,659 and 2,754,068, Japanese Patent Laid-Open No. 8-088765, and the like are known.
More specifically, another energy distribution corresponding to the radiation distribution is scanned in a direction perpendicular to the grid stripes to convert the grid stripes into a periodic signal on a video signal, and the analog periodic signal undergoes low-pass filtering and sampling on the time axis. With such normal arrangement of an antialiasing filter, inappropriate grid stripes can be removed.
As a method similar to the aforementioned method, U.S. Pat. No. 2,507,659 has proposed a method for detecting the presence and frequency of a grid stripe pattern image by computing the Fourier transform of a preliminarily sampled image, selecting a low-pass filter based on the detection result, and making low-pass filtering using the selected filter to remove inappropriate grid stripes.
U.S. Pat. No. 2,754,068, Japanese Patent Laid-Open No. 8-088765, and the like have proposed a method of obtaining an image at a desired sampling interval by making sampling on the time axis at an interval shorter than a desired interval in place of analog low-pass filtering proposed by U.S. Pat. No. 2,507,659 to capture image information containing a grid image by removing aliasing of grid stripe pattern information, making digital low-pass filtering that removes grid image components, and then digitally decimating (sub-sampling image information.
(Method 2)
In this digital conversion method, the radiation intensity distribution is temporarily converted into another energy distribution (that of, e.g., fluorescence, electric field strengths, or the like), which directly undergoes two-dimensional sampling using a plurality of electrical signal conversion elements (photodiodes, capacitors, or the like) in a two-dimensional matrix, and signals sequentially output from the respective conversion elements are A/D-converted.
As a typical one of such digital conversion method, a method using a so-called radiation flat panel sensor, i.e., a method of converting a fluorescence distribution or electric field strength distribution of radiation over a large area into electrical signals using a plurality of conversion elements for respective pixels in a large-screen flat sensor, whose technology has been developed in recent years, is known.
Upon capturing a digital radiation image by the digital conversion method of method 2, it is very difficult to remove an inappropriate grid stripe pattern due to, e.g., aliasing. This is because an antialiasing filter for an analog electrical signal cannot be applied unlike in method 1, since an energy distribution is directly sampled in a two-dimensional space using a plurality of electrical signal conversion elements of a radiation flat panel sensor or the like (to be simply referred to as a “sensor” or “flat panel sensor” hereinafter).
To solve this problem, a method of directly sampling an energy distribution using a sensor at a density which is high enough not to cause any aliasing in a two-dimensional space, and making the aforementioned sub-sampling after a digital antialiasing filter is applied may be used. However, in such method, it is difficult to make high-density sampling in the two-dimensional space due to the arrangement of the electrical signal conversion elements of the sensor, resulting in a considerable increase in cost.
Hence, a method of moving the grid during X-ray exposure is adopted as in the conventional system.
As another method, Japanese Patent Laid-Open No. 9-75332 or the like has proposed a method of preventing inappropriate grid stripes from being generated on an image by completely matching the spacing of grid stripes with the sampling pitch (pixel pitch of the sensor) to match areas where direct rays are intercepted by the grid stripes with pixel gaps for the purpose of removing the grid stripes upon capturing a digital X-ray image by directly sampling the energy distribution using a sensor in a two-dimensional space.
On the other hand, Japanese Patent Laid-Open No. 9-78970, U.S. Pat. No. 5,801,385, and the like have proposed a method of reducing the contrast of grid stripes by setting the spacing of the grid stripes to be smaller than the sampling pitch to be equal to or nearly equal to the width of the aperture of a light-receiving portion of one pixel (one electrical signal conversion element).
U.S. Pat. No. 5,050,198 or the like has proposed a method of removing a grid image by pre-storing images of grid stripe patterns (grid images) under a plurality of photographing conditions, and dividing an image obtained by photographing by one of the plurality of pre-stored grid images, which was obtained under the same or similar photographing condition.
However, the aforementioned conventional radiation image processes and, especially, an image process that captures a radiation image by (method 2), i.e., using a sensor that makes direct sampling in a two-dimensional space and a grid, suffer the following problems.
In the arrangement proposed by Japanese Patent Laid-Open No. 9-75332 or the like, it is very difficult to completely match the spacing of grid stripes with the sampling pitch. That is, a flat panel sensor which is normally manufactured in a semiconductor manufacturing process, and a grid formed as a combination of relatively thick lead plates are independently prepared, or the grid itself must be prepared to be detachable depending on a situation. Hence, it is very difficult to completely match the spacing of grid stripes with the pixel pitch (sampling pitch) of the sensor due to these factors.
On the other hand, in the arrangement proposed by Japanese Patent Laid-Open No. 9-78970, U.S. Pat. No. 5,801,385, or the like, it is effective to set the spacing of grid stripes to be smaller than the sampling pitch so as to be equal to or nearly equal to the width of the aperture of the light-receiving portion of one pixel. However, when the sensor (flat panel sensor) has high density and the sampling pitch becomes, e.g., 0.1 mm or less, a very small spacing of grid stripes (e.g., 10 stripes per mm) is required. In order to form a grid with such fine stripes, since the thickness of the lead plate used to intercept scattered rays is nearly fixed, areas where direct rays are transmitted through must be narrowed down. As a result, the use efficiency of the radiation dose drops very much, thus disturbing satisfactory radiography.
In the aforementioned conventional arrangement, the grid itself is moved during radiation exposure. Upon moving the grid, a drive system or the like used to move the grid results in an increase in cost and a bulky system, and an arrangement for adjustment control or the like of the relationship between the drive timing and radiation exposure timing, the relationship of the drive speed, and the like must be provided. Therefore, the arrangement that moves the grid cannot always be adopted although it is effective for removing grid stripes as it has the aforementioned limitations.
To solve the above problems, a method of removing grid stripes by digital filtering since the obtained radiation image is digital data may be used. With this method, if the spatial frequency of the grid stripes is completely separated from the spatial frequency components of effective image information based on an object, the grid stripes can be removed by a simple filtering arrangement.
As an example of this method, Japanese Patent Laid-Open No. 3-12785 or the like has proposed a method of removing or reducing data corresponding to the spatial frequency of grid stripes using Fourier transformation.
Also, a method of removing or reducing data corresponding to the spatial frequency of grid stripes using a normal FIR (Finite Impulse Response) filter has been proposed.
Since the grid stripe image is a shadow formed as a result of reducing the radiation transmittance by an X-ray shielding material such as lead or the like, it is multiplicatively superposed on a signal, but it is additively superposed if log conversion is done. Hence, the aforementioned filtering can be made.
In general, the manufacturing process of the grid used to remove scattered rays is managed with very high precision, and grid stripes having uniform spatial frequency characteristics for all kinds of images are prevalently used. For this reason, the aforementioned filtering may be done for only the single spatial frequency.
In practice, since the shape of the grid stripe image (shadow) is not an accurate sine wave shape, double, triple, . . . spatial frequency components as integer frequency multiples may be present. In this case, fundamental wave components alone may be received due to blur resulting from two-dimensional dependency of the conversion process (energy conversion process) of the sensor.
However, a problem with the aforementioned filtering is that it is nearly impossible to limit the spatial frequency band of the image components themselves.
More specifically, for example, as represented by arrangements proposed by U.S. Pat. No. 2,754,068, Japanese Patent Laid-Open No. 8-088765, and the like, grid stripes can be obviously removed by normal filtering without posing any problem as long as a very small spatial sampling pitch is set, an effective Nyquist frequency is increased after sampling to broaden the effective bandwidth (the bandwidth equal to or lower than the Nyquist frequency), and image components and grid stripe components can be perfectly separated in that band. However, it is not effective to decrease the spatial sampling pitch for only the purpose of removing grid stripes since it leads to very high cost of the sensor due to factors such as a semiconductor process and the like, and results in radiation capture efficiency drop.
Hence, it is effective to arrange the sensor itself with a spatial sampling pitch at which effective image components can nearly fall within a frequency band equal to or lower than the Nyquist frequency in terms of cost and performance. However, with this arrangement, the spatial frequency components of the grid stripes and effective image components inevitably overlap each other to some extent.
More specifically, such problem will be explained below using, e.g., FIGS. 40A to 40D. FIG. 40A shows an image signal when an image to be processed (source image) is observed one-dimensionally, and that image signal is made up of 256 numerical values.
FIG. 40B shows the response characteristics of a filter in the spatial frequency domain upon filtering the image signal shown in FIG. 40A. In FIG. 40B, the frequency domain is expressed by numerical values ranging from “0” to “128” in consideration of discrete Fourier transformation, and FIG. 40B expresses trap filtering at the position of the spatial frequency value=“100”.
FIG. 40C shows the result of filtering shown in FIG. 40B of the image signal shown in FIG. 40A. As can be seen from FIG. 40C, the characteristics of the image signal shown in FIG. 40C are nearly equal to those of the image signal shown in FIG. 40A.
FIG. 40D shows the difference between the image signals shown in FIGS. 40A and 40C for the purpose of confirmation. As can be seen from FIG. 40D, almost no signal components are removed by filtering.
FIG. 41A shows an image signal formed by adding a steeply rising middle portion (so-called edge portion) to the image signal (source image signal) shown in FIG. 40A.
FIG. 41B shows, as in FIG. 40B, the response characteristics of a filter in the spatial frequency domain upon filtering the image signal shown in FIG. 41A.
FIG. 41C shows the result of filtering shown in FIG. 41B of the image signal shown in FIG. 41A. As can be seen from portions bounded by circles in FIG. 41C, the filtering result unstably oscillates (artifacts) while being deviated from the source image signal.
FIG. 41D shows the difference between the image signals shown in FIGS. 41A and 41C for the purpose of confirmation. As can be seen from FIG. 41D, many oscillation components appear in portions that vary steeply (including the two end portions of the signal).
As shown in FIGS. 40A to 40D and FIGS. 41A to 41D, in case of a normal image signal, since considerably high-frequency components equal to or lower than the Nyquist frequency (spatial frequency=“128” in these figures) are not major components of the image signal and have nearly no information, if steep filtering is done at that position, no serious problem is posed. By contrast, when an image signal has steep portions (edge portion), since an image signal is expressed using considerably high-frequency components equal to or lower than the Nyquist frequency, problems (artifacts) are posed in portions that vary steeply.
FIGS. 42A to 42D show a signal state when a sine wave (sin(2π100χ/256)) is added to the source image signal shown in FIG. 40A upon simulating the grid. As can be seen from FIGS. 42A to 42D, grid stripes are nearly completely removed by filtering having the filter response characteristics shown in FIG. 42B (see FIG. 42C).
FIGS. 43A to 43C show a signal state when a sine wave (sin(2π100χ/256)) is added to the source image signal shown in FIG. 41A upon simulating the grid. As can be seen from FIGS. 43A to 43C, artifacts similar to those in FIG. 41C are generated by filtering having the filter response characteristics shown in FIG. 43B (see FIG. 43C).
That is, if grid stripe components are removed by a simple filtering process described in Japanese Patent Laid-Open No. 3-12785 or the like, the aforementioned artifacts may be generated intensely. If the impulse response width of the filter is narrowed to reduce artifacts, the response characteristics of filtering are reduced over a broad range, thus forming a strongly blunted image.
The present invention has been made to remove the above drawbacks, and has as its object to provide an apparatus, a system, a method, a program and a computer-readable storage medium storing the program, which can obtain a satisfactory radiation image of an object, substantially free from image components due to a grid, from a radiation image obtained by radiography using the grid.